The most common and most likely answer to this crossword clue is the 5 plus 5 letter word OMAHA BEACH
O
M
A
H
A
B
E
A
C
H
Buddy explains!
The straight part
We know that "D-Day code name" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case d-day code name = omaha beach.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.